Ceramic materials are of critical importance for a number of high temperature, high performance applications such as gas turbines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness. Design reliability and the need for economical fabrication of complex shapes, however, have prevented ceramic materials from fulfilling their potential in these critical high temperature, high performance applications.
The design reliability problems with ceramics, and the resultant failure under stress, are due largely to the relatively brittle nature of ceramics. This, in combination with the high cost of fabricating complex shapes, has limited the usage of ceramics.
Ceramics made from organosilicon polymers have the potential to overcome these problems. To this end, polymers based on silicon, carbon and/or nitrogen have been developed. See, for example, "Siloxanes, Silanes and Silazanes in the Preparation of Ceramics and Glasses" by Wills et al, and "Special Heat-Resisting Materials from Organometallic Polymers" by Yajima, in Ceramic Bulletin, Vol. 62, No. 8, pp. 893-915 (1983), and the references cited therein.
The major and most critical application for ceramics based on polymer processing is high strength, high modulus, reinforcing fibers. Such fibers are spun from organosilicon preceramic polymers, and then cured and pyrolyzed to their ceramic form. The low molecular weight and highly branched structure of typical preceramic polymers, however, alters the spinning and subsequent fiber handling behavior of these polymers from that of conventional polymers. For example, gelation and foaming tendencies in the melted polymers used for melt spinning may lead to the presence of undesirable flaws in the resulting fiber. Such flaws are undesirable in fine diameter fibers since they are believed to be the source of cracking and lowered tensile strength. Furthermore, because of the low molecular weight of the preceramic polymers used, the fibers spun therefrom have relatively low tensile strength and are difficult to handle in spinning, curing and subsequent pyrolysis operations. In addition to the thermal sensitivity of these organosilicon preceramic polymers, another problem in their melt spinning or extrusion is friability, i.e., the polymer chips are brittle and fragile, easily becoming crumbly, powdery or pulverulent upon physical handling, in contrast to chips of conventional polymers which will yield or deform. When subjected to compressive forces encountered in an overloaded transport screw such as those used in screw-type extruders, organosilicon preceramic polymer chips tend to produce powdery fines which deposit easily in crevices, thus compacting and becoming trapped in areas such as the screw flights. This problem is aggravated by the tendency of such fine particles of organosilicon preceramic polymers to physically adhere to metal surfaces as they soften. In typical screw-type extruders, this allows the trapped polymer particles to become prematurely heated, with the result that the polymer crosslinks, forming solid plugs that clog the extruder Also, regardless of whether the clogged extruder ceases to function, the increased melt residence time for the polymer in the extruder leads to thermal degradation and the production of preceramic and ceramic fibers of lowered tensile strength, thus increasing fiber handling difficulties.
Thus, the search has continued for improvements in these non-conventional melt spinning and fiber handling areas of ceramic fiber technology. The present invention was made as a result of this search.